Using Alpha Textures doesn't work properly
In your screen shot you plugged the alpha channel from the image in the the mix Shader.

Just plug the color socket of the image texture in to the factor of the mix Shader.

Edit:

The alpha channel of an image stores where the image is clear. The image in your question does not have an alpha channel, so it will be all white. If you used a png with transparency in the image then you would use the alpha channel as the mix factor.

If you change your image, delete all the black part so that it is clear. Then the alpha and color channels would be the same. White only where the letter P is.